The present invention generally relates to methods and apparatuses for performing thermal transfer recordings, and more particularly to a method and apparatus for accurately performing a thermal transfer recording (printing) according to a frame sequential system with respect to recording positions of three colors.
Conventionally, as systems for performing a thermal transfer recording, there are systems such as a frame sequential recording system, a line sequential recording system, a dot sequential recording system, and the like. Basically, the frame sequential recording system may be divided into a multi-path type which employs a single thermal printing head (TPH, hereinafter simply referred to as a thermal head) and performs the recording with respect to each of three colors, and a single-path type which employs three thermal heads and simultaneously performs the recording with respect to the three colors. Between the multi-path type and the single-path type, the thermal transfer recording which uses the frame sequential recording system of the multi-path type is being reduced to practice, and the present invention relates to an improvement of such a thermal transfer recording system.
A conventional recording apparatus which employs the thermal transfer recording system of the type described before, uses an ink ribbon. The ink ribbon comprises a base film on which solid ink having the three primary printing colors of yellow, magenta, and cyan, are successively and repeatedly coated on the base film in respective ink parts over a predetermined area along the longitudinal direction of the base film. A mark is provided on one side edge of the ink ribbon where the base film is not coated with the ink. The ink parts coated with the ink of the three primary colors constitute a group, and the mark is provided on the ink ribbon at a position corresponding to a starting end of each group. For example, the mark is provided on the ink ribbon at a position corresponding to a starting end of the yellow ink part in each group. In the recording apparatus, a recording paper which is an ordinary paper, is wound around a platen. The thermal head is arranged at a position opposing the platen, with the ink ribbon interposed between the thermal head and the platen. The thermal head extends along the width direction of the recording paper and the ink ribbon, and comprises a predetermined number of heating resistor elements, which predetermined number is equal to a number of picture elements constituting one line which is to be recorded. Position sensors are provided in the recording apparatus, at such positions that the mark on the ink ribbon is detectable by the position sensors.
When the position sensors detect the mark on the ink ribbon, the thermal head is heated after a time delay which is equal to a time it takes for the detected mark to reach a position in correspondence with the thermal head, and an information is first recorded with the yellow ink on the recording paper which is wound around the platen which rotates. In this state, predetermined heating resistor elements among a large number of heating resistor elements of the thermal head, are heated responsive to a recording signal. As a result, the predetermined heating resistor elements which correspond to the picture elements which are to be recorded, heat and melt the yellow ink on the yellow ink part of the ink ribbon and transfer the melted yellow ink into the recording paper. The platen is intermittently rotated by a step motor in synchronism with the feeding of the ink ribbon, and the recording with the yellow ink is performed in terms of one line. The recording with the yellow ink is performed along the longitudinal direction of the recording paper, and a predetermined number of lines are recorded on the recording paper. The length of each ink part of the ink ribbon (that is, the length of the ink part of the ink ribbon along the longitudinal direction thereof for each color), is set equal to the circumference of the platen. Thus, a starting end of a subsequent color ink part of the ink ribbon, the magenta ink part, for example, assumes a position in correspondence with a starting end of the recording paper which is pre-recorded with the yellow ink. Next, the information is recorded with the magenta ink on the recording paper which is pre-recorded with the yellow ink. Similarly, the information is recorded with the cyan ink on the recording paper which is pre-recorded with the yellow ink and the cyan ink, and the recording with the ink of the three primary colors with respect to the same recording paper is completed in this manner.
According to the conventional recording apparatus described heretofore, the recording of the information with the yellow ink is completed as the platen undergoes one revolution. The information is recorded with the subsequent magenta ink as the platen undergoes another revolution, and the information is recorded with the cyan ink as the platen undergoes still another revolution. However, it is extremely difficult to record the information with the ink of the three primary colors on exactly the same line, and there is a problem in that the recordings made by the ink of the three primary colors may not perfectly overlap with one another. In other words, the recording positions of the recordings made by the ink of the three primary colors may easily deviate from one another.
This problem may be overcome to a certain extent by improving the mechanical precisions of the platen, a mechanism for feeding the ink ribbon, and the like. However, there is a limit in improving the mechanical precision. The recording paper may shift from the clamped position on the platen as the platen rotates, and the recording paper may expand or contract due to the effects of the ink and the thermal head, depending on the quality of the recording paper. Moreover, the platen itself may become deformed after numerous recording operations. Generally, the recording positions of the recordings made by the ink of the three primary colors deviate from one another due to these causes when no measures are taken against these causes so as to control the recording positions of the recordings made by the ink of the three primary colors.